[ELF] Fix --soname option.

Currently LLD accepts only "-soname <string>", but all the following
options are actually valid.

  --soname=foo
  --soname foo
  -soname=foo
  -soname foo
  -h foo

This patch fixes that issue.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205662 91177308-0d34-0410-b5e6-96231b3b80d8